Panarthropoda
An arthropod is an having an (external ), a body, and paired jointed s. Arthropods form the Euarthropoda, which includes s, s, s, and s. The term Arthropoda as originally proposed refers to a proposed grouping of Euarthropods and the phylum . Arthropods are characterized by their jointed limbs and made of , often with . The arthropod consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by . Arthopods are bilaterally symmetrical and their body possesses an . Some species have wings. Their versatility has enabled them to become the most species-rich members of all in most environments. They have over a million described species, making up more than 80 percent of all described living animal species, some of which, unlike most other animals, are very successful in dry environments. Arthropods range in size from the microscopic crustacean up to the . Arthropods' primary internal cavity is a , which accommodates their internal , and through which their – analogue of – circulates; they have . Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their is "ladder-like", with paired running through all segments and forming paired in each segment. Their heads are formed by fusion of varying numbers of segments, and their s are formed by fusion of the ganglia of these segments and encircle the . The and systems of arthropods vary, depending as much on their environment as on the to which they belong. Their vision relies on various combinations of s and pigment-pit : in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information, but the main eyes of s are ocelli that can form images and, in a few cases, can swivel to track prey. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many bristles known as e that project through their cuticles. Arthropods' methods of reproduction and development are diverse; all terrestrial species use , but this is often by indirect transfer of the sperm via an appendage or the ground, rather than by direct injection. The evolutionary ancestry of arthropods dates back to the period. The group is generally regarded as , and many analyses support the placement of arthropods with ns (or their constituent clades) in a superphylum . Overall, however, the relationships of animals are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated. Aquatic species use either internal or . Almost all arthropods lay eggs, but s give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and s that lack jointed limbs and eventually undergo a total to produce the adult form. The level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by scorpions. Arthropods contribute to the human food supply both directly as food, and more importantly indirectly as of crops. Some species are known to spread severe disease to humans, , and crops. Phylogenetic tree | cladogram= | 1= |label2= |2= |label2=Euarthropoda |2= ' |label2= |2= ' |label2= |2= a, , , |barbegin1=brown |2= a, , |bar1=brown |2= , |bar1=brown |label2=' ' |barend1=brown |2= (springtails) |2= (coneheads) }} |2= (two-pronged bristletails) |2= }} }} }} }} }} }} }} }} }} }} }} References Category:Tree of life